What caused the deaths of the 129 sailors in the Franklin Expedition to the Arctic in 1845? Was there alien involvement in their fate, or was it something prosaic like food poisoning or polar bear attacks?

There was alien involvement here. There were a number who were taken, and eventually taken off planet. This was considered an intrusion as they had a large Arctic base and simply wanted to discourage human exploration of the entire region for reasons of security, as well as to interfere with human plans, which they do on a regular basis for all kinds of human endeavors, to throw a wrench in the works and cause things to fail that would add to human knowledge and encourage further exploration and expansion. They want to diminish humanity, worsen its capabilities, and discourage innovation and any kind of advancement of capabilities. There is nothing like causing an exploration to fail dismally to dampen enthusiasm for mounting such expeditions.